Light sources should have, besides high light output, good color rendition characteristics and selectable color temperature. An additional desirable characteristic is high stability during the normal lifetime of the lamp and during operation, insensitivity of the lamp with respect to tolerances during manufacture, and variations of operating conditions, for example variations in supply voltage.
It is known that electrical high-pressure discharge lamps are highly sensitive to changes in the above additional characteristics and, coupled therewith, to changes in electrical operating parameters. The sensitivity depends, to some extent, on the selected discharge medium, and the structural peculiarities of the particular lamp. This sensitivity to variations and to manufacturing tolerances interferes with many applications for such lamps; under particularly difficult conditions, or upon concurrence of a number of parameters of the lamp, or of its operation, failure of the lamp may result. One of the reasons for such variations are deviations which may arise during manufacture in the fill and of the geometry of the discharge arc. Another, and particularly important reason for the deviation are changes in the composition of the gas and/or of the pressure within the discharge vessel in the course of operation of the lamp, during its rated lifetime. These changes may be the result of chemical reactions of the various fill components among each other as well as with the materials of the discharge vessel. Loss of material and change in the gas composition may also be based on diffusion processes through the walls of the discharge vessel.
Variations in operating characteristics may also cause changes in temperature profiles and temperature distribution within the discharge vessel. The changes in the temperature may be the result of variation in lamp power or supplied energy due to variations in the voltage of the supply network. They also may be due to changed absorption characteristics and radiation characteristics of the components of the arc tube due to deposits or chemical changes or, even, due to temperature variations in the ambient surrounding of the lamp, and its fixture. Components of the gases, or the atmosphere within the
pressure lamp, which are in vapor pressure balance with a base body or element in the discharge vessel change if the vapor pressure changes. These changes in vapor pressure substantially influence the luminous flux as well as its spectral distribution. They also substantially vary the electrical characteristics of the lamp which, due to then changed energy balance of the lamp within its supply circuit, then influences the temperature distribution in the discharge vessel or arc tube of the lamp even further.
Temperature variations, which change the color rendition index and the light output, and variations in operating voltage may extend to such a level that the lamp may extinguish. This is particularly the case with sodium high-pressure lamps which are operated under saturated vapor condition, and which have a base body including sodium or sodium amalgam. Such changes are particularly annoying if the desired value of the color rendition index and, hence, the sodium vapor pressure, is high, and, as these values increase, the variations affect the operation of the lamp even more. In a standard sodium high-pressure lamp, having a color rendition index Ra of between 20-40, variations in color rendition index are not noticeable. The light output varies only slightly. The arc tube or running voltage rises only to impermissible values after the end of the rated lifetime. Such changes, however, have serious consequences in lamp types with improved color rendition indices, for example with a color rendition index Ra=60.
High-quality interior room illumination with sodium high-pressure discharge lamps requires operation of such lamps with high amalgam vapor pressures. A broadening of the resonance lines due to the partial pressures of the sodium and mercury components results in light having a color rendition index of Ra of about 80, with a color temperature of 2500 K. Different thermal conditions in the arc tube or discharge vessel itself and in its surroundings, as well as changes of the amalgam relationship due to diffusion of sodium and corrosion, lead to undesired changes in the color temperature, and the color locus on a color diagram, as well as to variations of the arc voltage upwardly or downwardly. If the arc voltage drops sharply, the lamp may even extinguish.
U.S. Pat. No. 5,103,141, Keijser et al (claiming priority of Netherland Serial 90 00531, filed Mar. 8, 1990, to which European Patent 445 882 corresponds) discloses control of the combination of running or arc voltage V and current I to a desired or command value C=V+.beta.I to stabilize the operation of such sodium high-pressure discharge lamps. This permits operation to maintain a "white" color locus at 2500 K. .beta. is a numerical factor which should be small. To compensate for changes in characteristics of the lamp during its rated lifetime, it is necessary, however, to suitably adjust and match the desired or command value C accordingly.
Sodium high-pressure discharge lamps with color temperatures of up to 2500 K can be operated by a conventional or an electronic current supply unit with continuous energy supply. For color temperatures above about 2500 K, a pulsed power supply for the lamp is necessary as described in the (former) East German Patent 270,405. Preferably, the fill of the sodium high-pressure discharge lamp does not contain any mercury but, rather, only sodium and a noble gas. With pulsed power supply, the energy supplied to the lamp is formed by a rapidly recurring sequence of high power short pulses, separated from each other by pauses during which low holding power is supplied, enough to prevent extinction of the discharge in the pauses between the high power pulses. Lamps can be operated with a thermal loading which is comparable to that of a standard lamp, while providing color temperatures of up to 3000 K, with a color rendition index of over 80, and supply relatively high light efficiency at its output. The color temperature is essentially determined by the instantaneous power of the lamp during the pulse phase; the color rendition index is determined primarily by the vapor pressure in the lamp. Investigations have shown, as illustrated in FIG. 1, that the color rendition index Ra can be raised up to Ra=80 by increasing the pressure with only slightly decreasing light output .eta., essentially indepently of the operating mode of the tamp. Further increase of the vapor pressure increases the color rendition index up to a maximum value of about 90 and, then, leads to a decrease to Ra=60. Coupled therewith is a substantial decrease in light output .eta. and substantial increase in the arc or running voltage, which might lead to extinction of the lamp. The equally important special color rendition index R.sub.9 for the red chromatic component, which is so important for interior illumination, rises with the vapor pressure to values of almost 100. In a region above Ra=85 it, however, drops rapidly and steeply to negative values.